SUNRPC: Dont run rpcauth_cache_shrinker() when gfp_mask is GFP_NOFS

Under some circumstances, put_rpccred() can end up allocating memory, so
check the gfp_mask to prevent deadlocks.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth.c

@@ -270,6 +270,8 @@ rpcauth_cache_shrinker(int nr_to_scan, gfp_t gfp_mask)
 	LIST_HEAD(free);
 	int res;
 
+	if ((gfp_mask & GFP_KERNEL) != GFP_KERNEL)
+		return (nr_to_scan == 0) ? 0 : -1;
 	if (list_empty(&cred_unused))
 		return 0;
 	spin_lock(&rpc_credcache_lock);